1. Field of the Invention
This invention relates to an actuator of piezoelectric driving system and an electronic hardware using it.
2. Background Art
MEMS (Micro-electro-mechanical System) actuator has been expected to be applied to various optical switches, communication circuits, variable-volume capacitors or micro-switches used in electronic hardware, and so forth.
Driving mechanism of MEMS actuator include a method of bending and carrying out displacement a beam by driving mechanism such as electrostatic force, thermal stress, electromagnetic force, and piezoelectric force. Among them, piezoelectric driving system has not only advantages of low power consumption and low driving voltage but also an advantage that the capacitance tuning ratio is large in the case of variable capacitor because the movable electrode does not perform pull-in to the fixed electrode by the driving voltage and therefore the actuator can be continuously and largely changed, and has particularly been focused.
However, the piezoelectric driving type actuator has a long thin beam structure in which a piezoelectric film is sandwiched by upper and lower electrodes, and therefore, the beam comes to bend up or down by a slight residual stress of the materials of the piezoelectric film or the upper or lower electrodes. For example, if the beam bends to the fixed electrode side, the movable electrode becomes in contact with the fixed electrode, and the MEMS variable capacitor indicates a high capacitance value and therewith the capacitance does not change, and the MEMS switch is in the ON state and therewith does not change to be in the OFF state. On the other hand, if the beam bends to the opposite side of the fixed electrode side, the movable electrode comes to detach from the fixed electrode, and in a general range of the driving voltage, the MEMS variable capacitor indicates a low capacitance value and therewith the capacitance hardly changes, and the MEMS switch is in the OFF state and therewith does not change to be in the ON state.
With respect to the problem of the beam warpage, an actuator having a folded beam structure has been proposed (JP-A 2006-87231 (Kokai)). However, the warpage of the beam of actuator is a spherical shape, and therefore, in this folded beam structure actuator, dissolution of warpage is insufficient. Therefore, when the valuable capacitor is produced by the actuator of this structure, the contact area of the movable electrode and the fixed electrode cannot be large and the maximum of the capacitance (maximum capacitance) is small. Moreover, change of the capacitance with respect to the applied voltage indicates precipitous change in the vicinity of the maximum capacitance, and control of the capacitance is difficult.